(a) Field of the Invention
The present invention relates to a self-sustained pulsation semiconductor laser device and, more particularly, to a self-sustained pulsation semiconductor laser device capable of operating at a low-noise.
(b) Description of the Related Art
A semiconductor laser device is widely used as a light source for an optical disk drive or an optical communication. In particular, the semiconductor laser device is recently highlighted for use in a high-density optical disk such as digital versatile disk (DVD) or magneto-optical (MO) disk. It is generally known that the semiconductor laser device used in such an optical pick-up system suffers from noise due to optical feedback from the optical disk surface. It is therefore important in the semiconductor laser device to suppress the optical feedback noise.
Driving the semiconductor laser device with a high frequency is known as one of the methods for decreasing the noise. Specifically, such a high-frequency drive of a semiconductor laser device provides multi-mode oscillation spectrum to thereby decrease the influence by the optical feedback. However, there arises in this technique a problem radiation of electromagnetic wave noise in addition to rise of the cost thereof, because of necessity for incorporation of a high-frequency superimpose module.
On the other hand, a self-sustained pulsation laser device has an advantage of absence of electromagnetic wave noise in addition to the advantages of low costs and a low-noise characteristic compatible with the high-frequency drive. Accordingly, a self-sustained pulsation semiconductor laser device having a low threshold current and a long-term reliability is especially desired.
The self-sustained pulsation can be obtained by incorporating a saturable absorber in the laser resonator to control the amount of the saturable absorption. Such a self-sustained pulsation and the laser structure therefor are reported in "Extended Abstract of 18th Conference on Solid State Devices and Materials", page 153, No. D-1-2, 1986, and in "Proceedings of 11th Semiconductor Laser Symposium", page 21, 1994. The semiconductor laser proposed in these papers has a saturable absorbing layer implemented by a portion of an active layer disposed under the current blocking layer. The proposed semiconductor laser has, however, a problem of a large astigmatism which is as high as between 10 .mu.m and 50 .mu.m due to a large optical absorption at the side of the mesa stripe.
Patent Publication JP-A-6-196810 proposes reduction of astigmatism by the configuration shown in FIG. 8, wherein a saturable absorbing layer is provided parallel to the active layer as a part of a cladding layer. In FIG. 8, n-AlGaAs cladding layer 103, n-type saturable absorbing layer 104, n-AlGaAs cladding layer 105, active layer 106, p-AlGaAs cladding layer 107, p-type saturable absorbing layer 108, p-AlGaAS cladding layer 109, p-GaAs cap layer 110, p-GaAs contact layer 111, n-GaAs current blocking layer 112 and a p-electrode are formed on the top surface of a GaAs substrate 102, with an n-electrode formed on the bottom surface of the GaAs substrate 102. An n-GaAs current blocking layer embeds p-AlGaAs cladding layer 109 and p-GaAs cap layer 110. In this configuration, a low optical feedback noise semiconductor device of a self-sustained pulsation and a small astigmatism is obtained by controlling the Al mole fraction (x) and thickness (between 0.01 .mu.m and 0.04 .mu.m) of n-Al.sub.x Ga.sub.1-x As saturable absorbing layer 104, and the composition (or Al mole fraction (x)) and thickness (between 0.01 .mu.m and 0.04 .mu.m) of p-Al.sub.x Ga.sub.1-x As saturable absorbing layer 108.
"IEEE Photonics Technology Letters", vol7, No12, pp1406, December 1995 describes a self-sustained pulsation semiconductor device, wherein a saturable absorbing layer, which is provided as a part of p-type cladding layer, is heavily doped with p-type dopant at as high as 2.times.10.sup.18 cm.sup.-3 carrier density, thereby achieving AlGaInP red laser lasing by self-excitation pulsation at temperatures up to about 50.degree. C.
"Proceeding of 43rd Meeting for Applied Physics Association", page 1024, 26a-C-10 describes a self-sustained pulsation lasing at a higher temperature and obtained by reducing the bandgap of a saturable absorbing layer compared to the bandgap of the active layer.
The semiconductor laser devices as proposed above have a disadvantage in that the yield (good products/total products) of semiconductor lasers capable of lasing by self-sustained pulsation is as low as on the order of 10%, according to experiments conducted by the inventor.